Congenital disorders of glycosylation (CDGs) are rare, screenable genetic disorders that deplete the synthesis of N-linked glycans. Mutations in PMM2, encoding phosphomannomutase 2 (PMM2, Man-6-P?Man-1-P) cause the most common form, CDG-Ia. Patients have a host of problems including hypotonia, variable psychomotor retardation, seizures, peripheral neuropathy, cardiomyopathy, and protein losing enteropathy. There is no therapy for this disorder. Other patients with CDG-Ib caused by mutations in phosphomannose isomerase (PMI) (Man-6-P<?Fructose-6-P) have milder symptoms, and they can be treated with dietary mannose supplements. Mannose is transported into cells, converted to Man-6-P, and bypasses the PMI deficiency. Note that both PMM2 and PMI compete for their common substrate, Man-6-P. Here the investigators propose a novel and unorthodox therapeutic approach to treat Pmm2-deficient cells and mice. By reducing PMI activity and providing exogenous mannose, they will try to drive more Man-6-P into the emaciated glycosylation pathway and rescue glycosylation. Usually, combining multiple disease-causing mutations within a biosynthetic pathway exacerbates the pathology, but not in this case. Preliminary results suggest that the investigators can rescue glycosylation of Pmm2-deficient cells and reverse embryonic lethality in a Pmm2-deficient mouse. They will simultaneously provide exogenous mannose and use both genetic and pharmacological methods to increase the availability and flux of Man-6-P into the glycosylation pathway. They developed viable and fertile mouse lines carrying hypomorphic alleles for Pmi and also identified several compounds that inhibit Pmi in cell-based glycosylation assays. Thus Specific Aim 1 will increase the flux of mannose into the glycosylation pathway of Pmm2 deficient cells by knockdown of Pmi or a specific Pmi inhibitor;Specific Aim 2 will determine to what extent, the specific Pmi inhibitor increases the flux of mannose into glycosylation pathway in normal mice;and Specific Aim 3 will introduce a hypomorphic Pmi allele into Pmm-deficient mice to rescue embryonic lethality at e18.5 days. This application has high impact because its success would validate a potential small molecule therapy for CDGIa patients. It may also create a flexible mouse model of CDG-Ia based on the addition or withdrawal of mannose from the drinking water. The high risk is that this approach will fail to rescue embryonic lethality or provide sufficient precursors for Pmm2 deficient cells. For CDG-Ia patients without any therapeutic options, the potential impact is worth the risk. PROJECT NARRATIVE: PMM2 deficiency causes congenital disorder of glycosylation (CDG) Type Ia, and no therapy exists. PMI deficiency causes the closely related CDG-Ib where dietary mannose therapy works. Both enzymes compete for the same substrate, mannose-6-P. The investigators predict that CDG-Ia patients will also respond to mannose supplements if the competing PMI activity is reduced. This application aims to validate that hypothesis.